Find the inflection points for the function: f(x) = 8x + 3 - 2sinx on the interval from 0 to 3pi. Give x values only and round to three decimal places.

Another example of a problem that I understand in general, but the specific problem throws me off...I differentiated twice and got f''(x) = 2sinx

I set that to zero and got sinx = -2

I assume this means there is an inflection point at -2, correct? This should be simple but trig functions always throw me off.